Magnetic field sensor systems are utilized in a variety of commercial, industrial, and automotive applications to measure magnetic fields for purposes of speed and direction sensing, angular sensing, proximity sensing, and the like. Magnetic field sensors may be based on semiconductor materials (e.g., Hall sensors, magnetoresistors, and so forth) and ferromagnetic materials (e.g., ferromagnetic magnetoresistors and flux guides). Other magnetic field sensors may utilize optical, resonant, and superconducting properties.
Tunnel magnetoresistance (TMR) sensor elements exploit a magnetoresistive effect that occurs in a magnetic tunnel junction (MTJ) structure. An MTJ structure includes a metal-insulator-metal layer sandwich in which the metal layers are ferromagnetic and the insulator layer is very thin. Electrically, this forms a tunnel diode in which electrons can tunnel from one ferromagnet into the other. Such a tunnel diode exhibits transport characteristics that depend, not only on the voltage bias, but also on the magnetic states of the top and bottom ferromagnetic layers.